The present invention relates to a fuel injector.
Ordinarily, changes in length of a piezoelectric actuator in a fuel injector are compensated for by the influence of temperature using hydraulic devices or by choosing suitable material combinations.
European Published Patent Application No. 0 869 278 describes a fuel injector in which the longitudinal change of the actuator is compensated for by an appropriate material combination. The fuel injector as in this document has an actuator, positioned in an actuator chamber, which is connected with form locking to a pressure shoulder via which the actuator acts upon the valve needle in opposition to the force of a pressure spring. The actuator is supported at one end on a pressure plate, and at the other end on a control element. During operation of the actuator, the valve needle is activated in the direction of spray-off.
In the document named, compensation for the longitudinal change of the actuator, caused by temperature, is achieved by a plurality of compensation discs positioned between the pressure plate and the end face of the actuator. These have a temperature expansion coefficient corresponding with opposite sign to that of the actuator element. During a shortening of the actuator caused by rising temperature, the compensation discs expand, and thereby compensate for the thermal longitudinal change of the actuator.
This design has a disadvantage above all in connection with cost of manufacture, having relatively high costs conditional especially on the choice of materials (e.g. INVAR). The compensation for longitudinal changes by hydraulic devices is known, for instance, from European Patent 0 477 400. With designs of this kind, the fundamental disadvantage is that large volumes of liquid have to be displaced, and, because of that, there is a greater tendency to cavitation.
The fuel injector according to the present invention on the other hand, has the advantage of simple construction of the component parts, from a standpoint of production engineering. This guarantees a fail-safe and precise method of operation of the fuel injector. Of particular advantage are the liquid support on both sides and the low damping volume for avoiding cavitational damage.
Especially of advantage are the encapsulation and prestressing of the actuator, since the quasi-static thermal linear deformation of the actuator does not have to be compensated for by costly material combinations, but is compensated for by a change in initial stress of the compression spring. Thereby, the overall length of the actuator housing is not influenced by thermal changes in length. For that reason, only a change in position of the actuator housing relatively to the valve housing still has to be compensated.
Sealing the actuator housing from the valve housing has the advantage that the actuator cannot be attacked by the chemically aggressive fuel.
The use of fuel as hydraulic medium is of advantage, since leakage losses can be compensated permanently by fuel supply.